Locales in available_locales should be lowercased

As I've made lowercase call in get_locale_by_name — all locales in dict should be lowercased also. 
I think it's better to handle any case in get_locale_by_name, then force developers to use "only correct case"
Merge pull request #16 from cleg/patch-2

Locales in available_locales should be lowercased